1. Field of the Invention
The invention relates to a microlens array used in optical systems, and in particular to a micolens array used in an image sensing device for correcting aberration such as light field curvature caused by the focal length differences of the microlenses therein.
2. Description of the Related Art
Solid state sensing devices, including charge coupled devices (CCD) and CMOS sensors, are commonly used in photo-imaging applications. A solid state sensing device includes an array of imaging pixels. Each of the pixels includes a photovoltaic device for converting light energy to electrical signals. The photovoltaic device can be a photogate, photoconductor, or a photodiode having a doped region for accumulating photo-generated charge.
Microlenses are commonly placed in a corresponding array over the imaging pixels. A microlens is used to focus light onto the charge accumulation region of the imaging pixels. Conventional technology forms microlenses from photoresist material which is patterned into squares or circles provided respectively over the imaging pixels. The patterned photoresist material is then heated during manufacturing to shape and cure the microlenses. Typically, the microlenses in the microlens array formed over the imaging pixels are of same radius, shape and height, and are arranged in fixed pitches. Use of microlenses significantly improves the photosensitivity and efficiency of the imaging device by collecting light from a large light collecting region and focusing it on a small photosensitive region of the pixel.
Nevertheless, as pixel size decreases and pixel density increases, use of the microlens array over the imaging pixels having microlenses of same radius, shape, height and fixed pitches have resulted in difficulty of the microlenses to focus incident light rays onto the photosensitive regions near an edge portion of the pixel array, thereby causing a light field curvature therein. This problem is due in part to incident light diffraction on the microlenses in different regions, thus causing variations of focal length of the microlenses in different regions, thereby forming image distortions in multiple regions above the photosensitive region.